Proanthocyanidins are tannins that are present in various plants. Proanthocyanidins are a group of compounds that are condensation or polymerization products (hereinafter, referred to as “condensation products”) and have flavan-3-ol and/or flavan-3,4-diol as a constituent unit. When these compounds are subjected to acid treatment, anthocyanidins such as cyanidin, delphinidin, and pelargonidin are produced. Therefore, these compounds are designated as proanthocyanidins.
Proanthocyanidins, which are one type of polyphenol, are potent antioxidants produced by plants, and are contained concentratedly in portions of plant leaves, bark, or skin or seeds of fruits. More specifically, proanthocyanidins are contained in, for example, the seeds of grape; the bark of pine; the skin of peanut; the leaves of ginkgo; the fruit of locust; and the fruit of cowberry. Moreover, it is known that proanthocyanidins are also contained in cola nuts in West Africa; the roots of Rathania in Peru; and Japanese green tea. Proanthocyanidins cannot be produced in the human body.
In recent years, it has been reported that among the proanthocyanidins, proanthocyanidins having a low degree of polymerization, and in particular condensation products having a degree of polymerization of 2 to 4 (dimer to tetramer), have excellent bioactivity. The condensation products having a degree of polymerization of 2 to 4 are generally referred to as oligomeric proanthocyanidins (OPCs).
Such proanthocyanidins are obtained by extracting from plants. As the extraction solvent, water; an organic solvent such as methanol, ethanol, acetone, hexane, and ethyl acetate; or a mixture thereof, is generally used (Japanese Laid-Open Patent Publication No. 11-80148). However, when simply performing an extraction with a solvent, the purity of the resultant extract is low and also the amount of proanthocyanidins that can be recovered is small. Therefore, in order to use the extract as raw materials for health food products, cosmetics, and drugs, it is necessary to increase the purity. Thus, additional processes such as concentration and purification are required, which increases the cost and time.
The following methods have been proposed as methods for recovering polyphenols containing proanthocyanidins. For example, Japanese Laid-Open Patent Publication Nos. 5-279264 and 6-56689 describe a process of adsorbing polyphenols to a chitin substrate and that the chitin substrate to which the polyphenols are adsorbed is utilized as a polyphenol product. Japanese Laid-Open Patent Publication No. 2002-97187 describes a method for recovering free polyphenols that comprises adding ascorbic acid and an alkali metal or a salt thereof to a plant extract liquid in order to adjust the pH in the range of 6 to 11, thereby precipitating a metal salt of polyphenols, recovering the metal salt, desalting the metal salt with an ion exchange resin or the like, and recovering the resultant free polyphenols.
However, most of the proanthocyanidins that can be obtained by any one of the above-described methods have a high degree of polymerization, and the content of OPCs (i.e., condensation products having a degree of polymerization of 2 to 4) that have excellent effects is very low.
Examples of methods for extracting OPCs from plants and/or synthesizing OPCs are disclosed in Japanese Laid-Open Patent Publication Nos. 4-190774, 10-218769, and 2001-131027; and Eberhard Scholz et al., “Proanthocyanidins from Krameria triandra Root”, Planta Medica, 55 (1989), pp. 379-384. However, in these extraction methods, an extract liquid of a plant is brought into contact with an adsorbent, the adsorbed material is eluted, and specific fractions are collected, and thereafter the same process is repeated using the collected fractions. Without repeating the process, an OPC-containing product having a high OPC content cannot be obtained, which is not efficient. The synthesizing methods also include a lot of steps, which causes problems of high cost and long time. Furthermore, the synthesizing methods also have a problem of liquid waste disposal.
Further, the inventors of the present application have disclosed that proanthocyanidin-containing product is obtained by the process of bringing an extract or juice of a plant into contact with a substrate made of chitin, chitosan, or a derivate thereof, recovering the resultant extract or juice containing materials that have not been adsorbed, and treating the resultant extract or juice with a synthetic resin adsorbent (WO03/090770); and that a pine bark extract with an increased proanthocyanidin content is obtained by the process of treating a pine bark extract with an adsorbent resin (Diaion HP-20, etc.) (Japanese Laid-Open Patent Publication No. 2005-47818).
However, there is a demand for a simple method for purifying proanthocyanidins containing a large amount of OPCs in addition to the above-mentioned methods.